The present invention relates to high frequency transistors and in particular relates to a high electron mobility transistor (HEMT) that incorporates nitride-based active layers and a silicon carbide substrate.
The present invention relates to transistors formed of semiconductor materials that make them suitable for high power, high temperature, and high frequency applications. As known to those familiar with semiconductors, materials such as silicon (Si) and gallium arsenide (GaAs) have found wide application in semiconductor devices for lower power and (in the case of (Si) lower frequency applications. These more familiar semiconductor materials have failed to penetrate higher power high frequency applications to the extent desirable, however, because of their relatively small bandgaps (e.g., 1.12 eV for (Si) and 1.42 for GaAs at room temperature) and relatively small breakdown voltages.
Accordingly, interest in high power high temperature and high frequency applications and devices has turned to wide bandgap semiconductor materials such as silicon carbide (2.996 eV for alpha SiC at room temperature) and the Group III nitrides (e.g., 3.36 eV for GaN at room temperature). These materials have higher electric field breakdown strengths and higher electron saturation velocities as compared to gallium arsenide and silicon.
A device of particular interest is the high electron mobility transistor (HEMT), which is also known as a modulation doped field effect transistor (MODFET). These devices offer operational advantages under a number of circumstances because a two-dimensional electron gas (2DEG) is formed at the heterojunction of two semiconductor materials with different bandgap energies, and where the smaller bandgap material has a higher electron affinity. The 2DEG is an accumulation layer in the undoped, smaller bandgap material and can contain a very high sheet electron concentration on the order of 1012 to 1013 carrier/cm2. Additionally, electrons that originate in the doped, wider-bandgap semiconductor transfer to the 2DEG, allowing a high electron mobility due to reduced ionized impurity scattering.
This combination of high carrier concentration and high easier mobility gives the HEMT a very large transconductance and a strong performance advantage over metal-semiconductor field effect transistors (MFSFETs) for high-frequency applications.
High electron mobility transistors fabricated in the gallium nitride/aluminum gallium nitride (GaN/AlGaN) material system have the potential to generate large amounts of RF power because of their unique combination of material characteristics which includes the aforementioned high breakdown fields, their wide bandgaps, large conduction band offset, and high saturated electron drift velocity. A major portion of the electrons in the 2DEG is attributed to pseudomorphic strain in the AJGaN; see, e.g., P. M. Asbeck et al., Electorinics Letters, Vol. 33, No. 14, pp. 1230-1231 (1997); and E. T. Yu et al., Applied Physics Letters, Vol. 71, No. 19, pp. 2794-2796 (1997).
HEMTs in the GaN/AlGaN system have been demonstrated U.S. Pat. Nos. 5,192,987 and 5,296,395 to Khan et al. (which are related as parent and divisional) describe HEMTs formed of a heterojunction between AlGaN and GaN on a buffer and a substrate. Other devices have been described by Gaska et al., xe2x80x9cHigh-Temperature Performance of AlGaN/GaN HFET""s on SiC Substrates,xe2x80x9d IEEE Electron Device Letters, Vol.18, No.10, October 1997 at page 492; and Ping et al., xe2x80x9cDC and Microwave Performance of High-Current AlGaN/GaN Heterostructure Field Effect Transistors Grown on P-Type SiC Substrates,xe2x80x9d IEEE Electron Letters, Vol.19, No.2, February 1998, at page 54. Some of these devices have shown fT values as high as 67 gigahertz (K. Chu et at., WOCSEMMAD, Monterey, Calif., February 1998) and high power densities up to 2.84 W/mm at 10 GHz (G. Sulllvan et a1., xe2x80x9cHigh-Power 10-GHz Operation of AlGaN HFET""s in Insulating SiC;xe2x80x9d IEEE Electron Device Letters, Vol. 19, No. 6, June 1998, pp. 198; and Wu et al., IEEE Electron Device Letters, Volume 19, No. 2, page 50, February 1998); Polyakou et al., xe2x80x9cThe Influence of Hydrogen Plasma Passivation on Electrical and Optical Properties of AlGaN Samples Grown on Sapphire,xe2x80x9dIII-Nitride, SIC and Diamond Materials for Electronic Devices Symposium, San Francisco, Calif., USA, Apr. 8-12, 1996, pages 607-611 xP000879409, 1996, Pittsburgh, Pa., Material Res. Soc. USA.
In spite of this progress, the gate peripheries corresponding to these results have beer too small to produce significant amounts of total microwave power with high efficiency and high associated gain. Thus the devices have tended to be of more academic than practical interest.
High power semiconducting devices of this type operate in a microwave frequency range and are used for RF communication networks and radar applications and offer the potential to greatly reduce the complexity and thus the cost of cellular phone base station transmitters. Other potential applications for high power microwave semiconductor devices include replacing the relatively costly tubes and transformers in conventional microwave ovens, increasing the lifetime of satellite transmitters, and improving the efficiency of personal communication system base station transmitters.
Accordingly, the need exists for continued improvement in high frequency high power semiconductor based microwave devices.
Therefore, it is an object of the present invention to provide a high electron mobility transistor (HEMT) that takes advantage of the electronic properties of Group III nitrides, and that does so in a manner superior to other existing and related devices.
The invention meets this object with a high electron mobility transistor (HEMT) that comprises a semi-insulating silicon carbide substrate, an aluminum nitride buffer Layer on the substrate, on insulating gallium nitride layer on the buffer layer, an active structure of aluminum gallium nitride on the gallium nitride layer, a passivation layer on the aluminum gallium nitride active structure, and respective source, drain and gate contacts to the aluminum gallium nitride active structure.
The foregoing and other objects and advantages of the invention and the manner in which the same are accomplished will become clearer based on the following detailed description taken in conjunction with the accompanying drawings in which: